Why splash is not in kernel parameters list but works?

This https://unix.stackexchange.com/questions/165110/grub-quiet-splash says: > The splash (which eventually ends up in your /boot/grub/grub.cfg ) > causes the splash screen to be shown. > > At the same time you want the boot process to be quiet, as otherwise > all kinds of messages would disrupt that splash screen. > > Although specified in GRUB these are kernel parameters influencing the > loading of the kernel or its modules, not something that changes GRUB > behaviour. However, I have not found splash on https://www.kernel.org/doc/html/v5.0/admin-guide/kernel-parameters.html , but AFAIK it works on modern distros which are kernel 5+ based. Why?